linux-kernel - Re: [PATCH 16/16 v6] PCI: document the new PCI boot parameters

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Date:	Fri, 07 Nov 2008 11:40:21 +0800
From:	"Zhao, Yu" <yu.zhao@...el.com>
To:	Greg KH <greg@...ah.com>
CC:	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"achiang@...com" <achiang@...com>,
	"grundler@...isc-linux.org" <grundler@...isc-linux.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"jbarnes@...tuousgeek.org" <jbarnes@...tuousgeek.org>,
	"matthew@....cx" <matthew@....cx>,
	"randy.dunlap@...cle.com" <randy.dunlap@...cle.com>,
	"rdreier@...co.com" <rdreier@...co.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"virtualization@...ts.linux-foundation.org" 
	<virtualization@...ts.linux-foundation.org>
Subject: Re: [PATCH 16/16 v6] PCI: document the new PCI boot parameters

Greg KH wrote:
> On Fri, Nov 07, 2008 at 10:37:55AM +0800, Zhao, Yu wrote:
>> Greg KH wrote:
>>> On Wed, Oct 22, 2008 at 04:45:31PM +0800, Yu Zhao wrote:
>>>>  Documentation/kernel-parameters.txt |   10 ++++++++++
>>>>  1 files changed, 10 insertions(+), 0 deletions(-)
>>>>
>>>> diff --git a/Documentation/kernel-parameters.txt 
>>>> b/Documentation/kernel-parameters.txt
>>>> index 53ba7c7..5482ae0 100644
>>>> --- a/Documentation/kernel-parameters.txt
>>>> +++ b/Documentation/kernel-parameters.txt
>>>> @@ -1677,6 +1677,16 @@ and is between 256 and 4096 characters. It is 
>>>> defined in the file
>>>>  		cbmemsize=nn[KMG]	The fixed amount of bus space which is
>>>>  				reserved for the CardBus bridge's memory
>>>>  				window. The default value is 64 megabytes.
>>>> +		assign-mmio=[dddd:]bb	[X86] reassign memory resources of all
>>>> +				devices under bus [dddd:]bb (dddd is the domain
>>>> +				number and bb is the bus number).
>>>> +		assign-pio=[dddd:]bb	[X86] reassign io port resources of all
>>>> +				devices under bus [dddd:]bb (dddd is the domain
>>>> +				number and bb is the bus number).
>>>> +		align-mmio=[dddd:]bb:dd.f  [X86] relocate memory resources of a
>>>> +				device to minimum PAGE_SIZE alignment (dddd is
>>>> +				the domain number and bb, dd and f is the bus,
>>>> +				device and function number).
>>> This seems like a big problem.  How are we going to know to add these
>>> command line options for devices we haven't even seen/known about yet?
>>> How do we know the bus ids aren't going to change between boots (hint,
>>> they are, pci bus ids change all the time...)
>>> We need to be able to do this kind of thing dynamically, not fixed at
>>> boot time, which seems way to early to even know about this, right?
>>> thanks,
>>> greg k-h
>> Yes, I totally agree. Doing things dynamically is better.
>>
>> The purpose of these parameters is to rebalance and align resources for 
>> device that has BARs encapsulated in various new capabilities (SR-IOV, 
>> etc.), because most of existing BIOSes don't take care of those BARs.
> 
> But how are you going to know what the proper device ids are going to
> be before the machine boots?  I don't see how these options are ever
> going to work properly for a "real" user.
> 
>> If we do resource rebalance after system is up, do you think there is any 
>> side effect or impact to other subsystem other than PCI (e.g. MTRR)?
> 
> I don't think so.
> 
>> I haven't had much thinking on the dynamical resource rebalance. If you 
>> have any idea about this, can you please suggest?
> 
> Yeah, it's going to be hard :)
> 
> We've thought about this in the past, and even Microsoft said it was
> going to happen for Vista, but they realized in the end, like we did a
> few years previously, that it would require full support of all PCI
> drivers as well (if you rebalance stuff that is already bound to a
> driver.)  So they dropped it.
> 
> When would you want to do this kind of rebalancing?  Before any PCI
> driver is bound to any devices?  Or afterwards?

I guess if we want the rebalance dynamic, then we should have it full -- 
the rebalance would be functional even after the driver is loaded.

But in most cases, there will be problem when we unload driver from a 
hard disk controller, etc. We can mount root on a ramdisk and do the 
rebalance there, but it's complicated for a real user.

So looks like doing rebalancing before any driver is bound to any device 
is also a nice idea, if user can get a shell to do rebalance before 
built-in PCI driver grabs device.

Regards,
Yu
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